1. Field of the Invention
This invention generally relates to a structure in which an electronic component and a base board are connected by an array of solder bumps provided on one surface of the electronic component. This invention particularly relates to an electronic-component mounting structure suited for use in a portable electronic apparatus.
2. Description of the Related Art
There are known techniques for increasing the life time of interconnecting solder bumps in BGA-package (ball grid array package) mounting structures.
Japanese published unexamined patent application 7-321247 discloses a BGA-package mounting structure. In Japanese application 7-321247, electrodes in an array on one side of a BGA package have elliptical shapes whose major axes correspond to dimensions along lines passing through the centers of deformations caused by thermal distortions. In addition, electrodes in an array on a base board have elliptical shapes substantially equal to those of the package electrodes. The package electrodes and the base-board electrodes are connected by solder balls. The BGA-package mounting structure in Japanese application 7-321247 suppresses the adverse affection of thermal distortions on the interconnecting solder balls, and thereby increases the life time of the interconnecting solder balls.
In general, a portable electronic apparatus is sometimes subjected to a mechanical shock. For example, when the portable electronic apparatus is accidentally dropped, such a mechanical shock occurs. Conventional solder bumps tend to be damaged by 5 mechanical shocks. Thus, it is desirable that portable electronic apparatuses use solder bumps which withstand mechanical shocks.
In the BGA-package mounting structure of Japanese application 7-321247, since the package electrodes and the base-board electrodes are substantially equal in shape, the solder bumps (the solder balls) have barrel-like shapes of FIG. 33 after a reflowing process is complete. The barrel-like solder bumps have such angles of contact with the electrodes that stresses tend to be applied to the bumps by mechanical shocks. Thus, it seems that the interconnecting solder bumps in the BGA-package mounting structure of Japanese application 7-321247 are easily damaged by mechanical shocks.
Japanese published unexamined patent application 9-45733 discloses a method of mounting electronic components. According to the method in Japanese application 9-45733, electrodes on an electronic component are connected to electrodes on a base board by solder bumps. After a reflowing process is complete, some of the interconnecting solder bumps have drum-like shapes with constricted middle portions as shown in FIG. 34. The constricted middle portions cause low strengths of the interconnecting solder bumps. Thus, the interconnecting solder bumps in Japanese application 9-45733 seem to be short in life time.